Food color is one of the critical factors for appetite stimulation. For example, a vividly colored food with no color change or fading is visually enjoyable; further, the beautiful appearance of the food increases the joy of eating. As such, food color can affect the palatability of food.
In consideration of such importance of food color, food coloring technologies have been drawing attention, with the recent growing market of processed food and preserved food. Food coloring agents include natural colorants derived from plant rhizome or fruits, and artificial colorants such as tar dye. Recently, people tend to prefer natural colorants with more natural tones.
Among conventionally known natural colorants is curcuminoid obtained from Curcuma longa L., which is a plant of the family Zingiberaceae. Curcuminoid is a general name for compounds including curcumin and an analog thereof, i.e., demethoxy curcumin and bis demethoxy curcumin, and is also a kind of polyphenol. Here, “curcuminoid” is a general name of linear diarylheptanoids such as curcumin, demethoxy curcumin, bisdemethoxy curcumin, yakuchinone A, yakuchinone B, tetrahydrocurcumin, or dihydroxy tetrahydrocurcumin, as well as the salts and esters of these linear diarylheptanoids and analog compounds of other linear diarylheptanoids, such as cassumunins A, B, and C, which are called curcumin polymers.
Curcuminoids have a yellow color, which is particularly vivid among natural yellow colors. Further, curcuminoids also have advantages including excellent heat resistance, oxidation resistance, and reduction resistance; and superior dying power with respect to animal and plant-derived substances. Curcuminoids can be used in, for example, curry powder, takuan pickles (yellow pickled radish), and mustards.
However, curcuminoids have a drawback in that although they are soluble in hot water, ethanol, propyleneglycol, and glacial acetic acid, they are poorly soluble in water (cold water or water at room temperature). Therefore, it is difficult to add yellow color to an aqueous substance using powdered curcuminoid.
In view of this problem, an aqueous turmeric pigment preparation was developed to enable coloring of an aqueous substance with a turmeric pigment, and a method using this pigment preparation was suggested (Patent Document 1). The aqueous turmeric pigment preparation is a so-called alcohol-soluble solution (hydroalcoholic extract solution) obtained by subjecting turmeric powder, which is prepared by pulverizing curcuma dried rhizome, to extraction using hydrous alcohol, and removing a residue (insoluble matter) by solid-liquid separation. Although the extract solution has an advantage in that curcuminoids are dissolved in a solution containing alcohol at a high color value, it suffers some drawbacks; e.g., the danger in the preparation step using flammable alcohol, the insoluble curcuma residues (waste substance) produced in the preparation step, and the particular taste and smell of the resulting extract solution that are derived from the bitter taste and pungent component of turmeric powder. This limits the use of the extract solution as a food colorant. Moreover, there is another drawback in that, when this extract solution is added to water so as to decrease the alcohol concentration, curcuminoids are precipitated over time; consequently, curcuminoids are insolubilized and settled out.
As another method of preparing a water-soluble curcuminoid pigment preparation, Patent Document 2 discloses a method of dispersing in water an alcoholic solution in which a refractory element material such as curcumin or tetrahydrocurcumin is dissolved, thereby first making fine particles of the refractory element material and then combining the obtained particles with an emulsifier (glycerin fatty acid esters, sucrose fatty acid esters, sorbitan fatty acid esters, propylene glycol fatty acid esters, polyoxyethylene sorbitan fatty acid esters, stearin lactic acid calcium, lecithins, cyclodextrin, and the like), thus obtaining a translucent solubilized liquid that is stable for a long period of time. Further, Patent Document 3 discloses a method of dissolving curcumin in hot water or alkaline aqueous solution, removing insoluble residues, and making the pigment finer by dyeing a fine cellulose substance with the resulting solution-form curcumin; thereby preparing a yellow food colorant composition that can easily be dispersed in a pasty aqueous substance or an oily substance, and thereby adding a vivid yellow color to the substance. Additionally, Patent Document 4 discloses a method for preparing a water-soluble transparent curcumin complex with improved light stability and coloring ability, the method comprising contacting curcumin with a substrate selected from a water-soluble branched-chain or cyclic polysaccharide and a water-soluble or water-dispersible protein in an alkaline aqueous solution at a pH of 9 or more, and then acidifying it to a pH of 8 or less. Patent Document 5 further discloses a method for preparing a curcumin-gelatin complex (water-soluble curcumin preparation) by dissolving gelatin and curcumin of 15 wt % at maximum in a solvent containing specific amounts of water and acetic acid, thereby preparing a curcumin-gelatin complex that can be dissolved in water without an artificial emulsifier and that is capable of making a transparent yellow solution.
On the other hand, in addition to the usages as a coloring agent or a dye, curcuminoids have also long been used as a choleretic drug. Further, recent studies found useful physiological activity of curcuminoids, including tumorigenesis inhibitory activity, antioxidant action, antiinflammatory effect, hypocholesterolemic activity, antiallergic action, and encephalopathy prevention activity. Thus, curcuminoids are expected to be applied to medicinal products, cosmetics, dietary supplements and the like (Patent Document 6, Non-Patent Documents 1 to 8).
However, as described above, curcuminoids are sparingly water-soluble; therefore, only a small portion thereof will be absorbed into the body upon oral administration. Thus, the aforementioned useful physiological activities of curcuminoids cannot be fully exhibited in the body. In view of this problem, Patent Document 6 suggests using water-soluble curcuminoid glycosides instead of the sparingly water-soluble curcuminoid, so as to improve the absorbability of curcumin into the body and sufficiently exhibit the superior medicinal effect of curcumin in vivo.
As explained above, the techniques disclosed in Patent Documents 1 to 6 are all directed to solubilization of turmeric pigment in water. However, the solubilization of turmeric pigment in water is actually limited, as it is saturated at a certain concentration. When the concentration reaches this point, any attempt to dissolve more turmeric pigment will result in the generation of aggregation and sedimentation of the turmeric pigment. As such, it is difficult to actually incorporate a turmeric pigment in an amount larger than the allowable content.